Cancer cells differ from their normal cellular counterparts in many important characteristics, including loss of differentiation, increased genomic instability, and decreased drug sensitivity. Not surprisingly, genetic alterations occur in most, if not all cancer cells, and are thought to lie at the heart of these phenotypic alterations. Furthermore, an underlying genetic instability is thought to be required to generate the multiple genetic changes that occur in cancer cells. My laboratory uses somatic cell and molecular genetics to identify and characterize genetic alterations found in tumor cells that induce abnormal cellular phenotypes. By utilizing this approach, my lab has identified a previously unknown chromosomal abnormality that is associated with certain chromosomal rearrangements. This chromosomal phenotype is characterized by a delay in mitotic chromosome condensation, a delay in the chromosome replication timing, over-expression of the replication checkpoint kinase ATR, and significant chromosomal instability. Chromosomes with this phenotype are common in tumor derived cell lines and in primary tumors. Furthermore, we have found that chromosomes with this phenotype activate the DNA replication checkpoint (S-M phase checkpoint). Our findings support a model in which the chromosomal instability found in tumor cells stems from a defect in the replication timing of certain chromosomal rearrangements. The experiments described in this proposal are designed to characterize the cellular response to these abnormal chromosomes. This proposal is designed to: 1) determine the mechanisms by which stalled or delayed DNA replication results in ATR over-expression; 2) characterize the role that the DNA replication checkpoint plays in the DRT/DMC phenotype, and 3) determine if chromosomes with DRT/DMC activate the mitotic spindle checkpoints. [unreadable] [unreadable]